1. Field of the Invention
Exemplary aspects of the present invention generally relate to an optical scanner and an image forming apparatus including the same, and more particularly, to an optical scanner using a resonance mirror.
2. Description of the Background Art
Conventionally, a known method of writing an image on a photoreceptor drum includes rotating a polygon mirror to reflect and scan a laser beam emitted from a light source. To accommodate increasing demand for high-speed and high-resolution imaging, the speed of rotation of the polygon mirror is increased, causing a rise in temperature, an increase in noise, and accelerated degradation of the polygon motor that drives the polygon motor. Thus, there is a limitation on how much the speed of rotation of the polygon mirror can be increased.
In the meantime, with advances in precision processing technologies such as Micro Electro Mechanical System (MEMS), an alternative to the polygon mirror has been proposed in the form of an optical scan method using the mechanical resonance of a micromirror having frequency transfer characteristics as shown in FIG. 8. Compared with the polygon mirror, the optical scan method is advantageous in that small size, low weight, low noise, low heat emission, and low power consumption can be achieved. For this reason, it is expected that MEMS will eventually replace the polygon mirror.
Nevertheless, in order to facilitate an understanding of the related art and of the novel features of the present invention, a description is now provided of a configuration of a related-art optical scanner using the polygon mirror with reference to FIG. 9. FIG. 9 is a schematic diagram illustrating such related-art optical scanner.
As illustrated in FIG. 9, the optical scanner includes a polygon mirror driver 216, a light source 211, a polygon mirror 212, and a light-receiving sensor 214. The light-receiving sensor 214 is disposed on a scan-start side of an effective scan area relative to the polygon mirror 212. In FIG. 9, a laser beam emitted from the light source 211 is reflected by the polygon mirror 212 and illuminates the surface of a photoreceptor drum 213.
Since the polygon mirror 212 rotates at a constant speed, the laser beam scans the effective scan area of the photoreceptor drum 213 at a constant angular speed. The light-receiving sensor 214 is provided at the scan-start side of the effective scan area to detect timing of the laser beam passing by.
The output of the light-receiving sensor 214 is supplied to a main controller 215. The main controller 215 controls the polygon mirror driver 216 of the polygon mirror 212 such that the speed of rotation of the polygon mirror 212 coincides with a target speed of rotation while obtaining information on writing timing.
By contrast, as the alternative method to the scan method using the polygon mirror as described above, there is growing interest in the micromirror because of its low heat emission and low power consumption. Compared with constant rotation of an inertial body such as the polygon mirror, the micromirror needs to be oscillated in a simple harmonic motion at a frequency near the resonance frequency. By contrast, in the case of the polygon mirror, since the laser beam moves at a constant angular speed, the linear velocity of the laser beam increases as the laser beam approaches the end portion of the surface of the photoreceptor compared to the center of the photoreceptor.
With the micromirror, the angle of the mirror changes in a sine wave manner along the time axis, so that the linear velocity of the laser beam decreases as the laser beam approaches the end portion of the photoreceptor surface, as compared with the center of the photoreceptor. Deviations in the linear velocity are corrected by a lens optical system.
Referring now to FIG. 10, there is provided a block diagram illustrating the configuration of the related-art optical scanner using the polygon mirror. The polygon mirror (the inertial body) including a plurality of mirror faces is rotated at a constant speed.
As illustrated in FIG. 10, the light-receiving sensor 214 detects the reflected light from the polygon mirror 212. Based on the output of the light-receiving sensor 214, a signal processor 223 converts the output to a true phase. Subsequently, the true phase is compared with a target phase by a comparator 221. Based on the difference in the phase, a phase controller 222 controls the polygon mirror driver 216 such that the speed of rotation of the polygon mirror 212 coincides with the target speed of rotation.
The optical scanner using the polygon scanner is advantageous in that an accurate rotation speed can be obtained by such a simple phase-locked loop (PLL) control. By contrast, in a case of the micromirror, due to its structure, the micromirror needs to be accurately oscillated in the simple harmonic motion. Thus, the method of rotating the inertial body at a constant speed cannot be applied to this configuration.
Furthermore, a drawback to this technology is that writing operation needs to be performed with precision relative to the photoreceptor by the simple harmonic oscillation of the micromirror, necessitating high-precision beam scanning. In particular, amplitude accuracy in a main scan direction is required for good imaging quality for a printer.
In general, such a resonance mirror is used under atmospheric pressure or reduced-pressure atmosphere conditions. Consequently, when the mirror is operated with simple harmonic oscillation at a frequency near the resonance frequency, the amplitude accuracy is subject to degradation due to disturbance around the mirror.
To counteract such a problem and achieve a desirable amplitude accuracy, some kind of feedback control is necessary. In order to provide a feedback control system at low cost, it is necessary to detect amplitude information in discrete time. Furthermore, since it is a system in which a sampling frequency of the amplitude information depends on a drive frequency, in reality, the response exhibits the closed-loop response of the discrete time control system relative to an ideal closed-loop response of the continuous-time control system.
FIG. 12 is a diagram illustrating an actual closed-loop response of a discrete time control system. C1 in FIG. 12 indicates complementary sensitivity function (target-follow performance), and C2 indicates sensitivity function (suppression performance). From FIG. 12, it can be seen that, compared to the ideal system, the ability to suppress disturbance is reduced (indicated by a dotted circle), and the desirable amplitude accuracy cannot be achieved.